Motor vehicles, such as tractors, are powered by combustion processes which take place in the cylinders of the engines of the vehicles. The processes in gasoline engines involve ignition of a gasoline and air mixture by a spark. In diesel engines, the processes involve a spontaneous ignition of a mixture of diesel fuel and highly compressed heated air.
For the spontaneous combustion of diesel fuels to take place, the air must attain a temperature of 700-900 degrees F. This is because a diesel fuel has a temperature flash point that is relatively high among fuels. Thus, under cold weather conditions, the engine environment may have such heat loss or the engine may not be able to attain a high enough compression to sufficiently heat the air to the combustible temperatures required to ignite diesel fuel.
It has been known to use an ancillary fuel as a starting fuel to start the combustion process. Ether has long been the preferred ancillary fuel. It has a low flash point that is an advantage in starting the engine. But it causes an engine to run hot once started, which may be harmful to the engine. Moreover, because of its low flash point, combustion takes place prematurely during the engine power cycle as the engine heats up, which results in the piston being driven against the bottom of the cylinder rather than against a lubricant cushion. This is particularly true under load conditions when there is power removed from the engine, as when the vehicle is in a driving gear. Both the hot running of the engine and the driving of the piston against the bottom of the cylinder can cause accelerated engine wear.
Notwithstanding the effects of ether use, the vehicle operator may inject ether into the engine to obtain added power. Consequently, at least two systems have been developed to prevent accidental or intentional injection of ether into the engine once the engine is running.
One system that has been in use for some time involves use of a thermal switch on the engine block. According to this system, once the engine has warmed sufficiently, the thermal switch heats up and opens so as to open the operating circuit between the ether switch and the valve operated by the ether switch and through which the ether is injected into the engine manifold.
This system used alone, however, has a considerable disadvantage under cold engine operating conditions when the engine block has not warmed enough to open the thermal switch. Ether still could be injected into the manifold under load conditions.
A more current approach is to connect the ether switch to the cranking circuit so that ether can be injected only during engine starting. While this gives added protection, it too has a disadvantage. Under certain cold starting conditions, the engine may require ether to keep the engine running after disengagement of the cranking motor. But as ether can no longer be injected, the engine "dies", with a loss of cold starting performance.